Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more beds of the same or different catalyst.
Slurry hydrocracking (SHC) is a slurried catalytic process used to crack residue feeds to gas oils and fuels. SHC is used for the primary upgrading of heavy hydrocarbon feed stocks obtained from the distillation of crude oil, including hydrocarbon residues or its mixture with gas oils from atmospheric distillation tower bottoms or vacuum distillation tower bottoms. Alternative sources of heavy hydrocarbon feed stocks include solvent deasphalted pitch or visbroken residues. In slurry hydrocracking, these liquid feed stocks are mixed with hydrogen and solid catalyst particles, e.g., as a particulate metallic compound such as a metal sulfide, to provide a slurry phase. Slurry hydrocracked effluent exits the slurry hydrocracking reactor at very high temperatures around 400° C. (752° F.) to 500° C. (932° F.). Representative SHC processes are described, for example, in U.S. Pat. Nos. 5,755,955 and 5,474,977.
One difficulty with SHC processes is the increased risk of reactor fouling when operating at higher pitch conversion. Pitch recycle can be used to reduce the potential for reactor fouling and is necessary to achieve high pitch conversion. Incompatibility can also occur in fractionation columns or other vessels when different hydrocarbon streams are contacted with each other and form immiscible liquid phases due to thermodynamic phenomena.
In an SHC process, recovery of pitch is typically achieved by a vacuum fractionation column that cooperates with an upstream series of separators, stripper(s) and an atmospheric fractionation column. Atmospheric and vacuum fractionation columns provide streams with specified boiling point ranges that are transported to downstream upgrading units. The vacuum fractionation column separates an unconverted pitch stream from vacuum gas oil (VGO). Other recovered products include naphtha, kerosene and diesel.
SHC products typically require additional heating before delivery to a vacuum fractionation column. To achieve low VGO concentration and high pitch concentration in the vacuum bottom stream, the fractionator feed stream and column vaporization zones require high temperature to make up for losses in vacuum pressure encountered while passing through vacuum packing materials along the height of the column. Packing materials are added to achieve efficient separation of product streams. High temperature increases the risk of severe fouling. Otherwise, poor separation can result in high concentrations of VGO or even lighter products in the vacuum column bottoms.
Improved fractionation of pitch from VGO is needed in SHC recovery processes.